A better way to make this car air vents holes?
Suggestions.  An increment in vertex density and use extrude.
The image above is an exaggeration of your subtle situation. Model progression.

Top Ortho.  


Loop Cuts.
Delete Vertices and faces.
Shape the hole manually, or with Proportional edit in Edit Mode, or with Loop Tools which is an addon.
Extrusion marked by arrow and difficult to see


User Persp


Extrusion allowing flexible shaping.  You are vents are somewhat like eyelids. They need less height. Add more loop cuts and shape subtly.




Still exaggerated for readability. Image above.  You can be more artistic with technical improvements.  This is an isolated modeling suggestion.  You can see models of hand and eyes to see how many loop cuts can be reused and reduced.